Creating correct proportions for Minecraft projects
I think this is more of a 2D/drawing question so I'll keep my answer very light.

It’s all in the references. You need to find a front view and a side view that are as close to actual size as possible. In other words they have to look similar to blueprints. Then you just need to measure, like with Gimp, and see the ratios. If the vertical is 10 and the horizontal 7 from the front view and 10 and, say, 5 in depth from the side, then you obviously have a 7 to 5 ratio from top view. As a note, if the objects are at different relative sizes, you can scale one of the images until the verticals coincide. It can be done by numbers too, but I don’t think is necessary to explain.

So ideally is to make the images yourself and get the desired angles. Proportions can be extracted from a  ¾ quarter view too, but, again, I don’t think this is a Blender topic, rather a drawing one.